The present invention relates to pulmonary function testers and more particularly to spirometers which measure air entering and leaving the lungs during different breathing
Various types of techniques have been used to measure respiratory flow in pulmonary function testers. One such technique determines air flow and volume by measuring the pressure differential created by air flowing across a resistance in a tube. For clinical evaluation it is desirable that a device be capable of measuring air flow volume in the range of 12 milliliters/second (ml/sec) to 12 liters per second (1/sec), which is a range of 1,000-to-1. For example, for a pulmonary function tester using an orifice as a res1stance element, the air flow to resistance differential has a square-law relationship. Therefore, it is necessary to measure pressure over a relatively large range. Furthermore, in clinical evaluation, it is necessary, according to ATS standards, for the pulmonary function tester to have a high accuracy on the order of 3%. Thus, clinical devices should not only be capable of measuring over a relatively large pressure range, but also provide accurate measurements.
Existing pulmonary testers often go out of calibration, i.e. their "zero level" shifts, resulting in inaccurate measurements. In one particular type of pulmonary function tester wherein a pressure transducer is used to detect pressure changes, the stability of the transducer output voltage at zero pressure input ("zero" or "zero level") becomes the most important performance characteristic, as either long term or short term drift adversely effects the operation and accuracy of the instrument. This is particularly so where the transducer manufacturer's zero drift specifications typically approach 1/3 of the maximum pressure which is created when the pulmonary function tester is in use. Thus, close attention to the zero level is important to achieving an overall 3% volume and flow measuring accuracy.